<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script type="text/javascript">
        var arr = [];
        //object
        console.log(typeof arr);
        arr[1] = false;
        //length = 2
        console.log(arr.length);
        //自动扩容时，之前未定义的数组项 = undefined
        arr[3] = 'abcd';
        arr[5] = 999;
        for (var i = 0; i < arr.length; i++) {
            console.log(arr[i]);
        }
    </script>
</head>
<body>

</body>
</html>